Linear sensor and preparation method thereof

Abstract

The linear sensor comprises a ferromagnetic layer, an antiferromagnetic layer and a heavy metal layer, the ferromagnetic layer is arranged on the upper surface of the antiferromagnetic layer, and the heavy metal layer is arranged on the upper surface of the ferromagnetic layer as a protective layer; during working, a Wheatstone bridge structure is adopted, the included angle between the magnetization pinning direction on each bridge arm and the long axis direction is 45 degrees, and the magnetic resistance response trends of the magnetic resistance on the adjacent bridge arms to the same external magnetic field are opposite; one-way current pulses with set amplitudes are introduced into the two ends of the magnetic resistor to complete the induction of exchange bias. According to the invention, through the selection of each layer of material of the sensor film, the design of the growth sequence and the design of the rear-end Wheatstone bridge, the regulation and control of the Wheatstone full-bridge structure sensor can be realized in a non-magnetic environment only through one-time current pulse. The growth efficiency of the magnetic sensor is greatly improved, the regulation and control steps are reduced, and the regulation and control procedure of the sensor is simplified.

Description

Technical Field

[0001] This invention belongs to the technical field of sensors and relates to a linear sensor and a preparation method thereof. Background Art In the application of AMR magnetic sensors, in order to obtain a good linear output of magnetoresistance, the magnetization direction of the magnetoresistance needs to be designed to form an angle of 45° with the current. A relatively traditional solution is to introduce Babinet electrodes, but this method reduces the effective area of the magnetoresistance. In addition, the current at the boundary part cannot flow into the magnetoresistance well at an angle of 45°, so the linear output ability and sensitivity of the sensor will be lost. If the ferromagnetic and antiferromagnetic phases are pinned and the magnetization direction is fixed at an angle of 45° with the long axis to obtain a linear output, a magnet is required as an auxiliary field to induce the formation of exchange bias during the preparation process, or after the sensor is manufactured, it needs to be annealed in a magnetic annealing furnace at a high temperature to destroy the ferromagnetic-antiferromagnetic exchange coupling state and annealed in a strong magnetic field environment to re-pin the magnetization in the required direction, and the procedure is relatively cumbersome, and the preparation efficiency of the sensor is low. Summary of the Invention

[0002] The purpose of this invention is to provide a linear sensor and a preparation method thereof to solve the above problems.

[0003] To achieve the above purpose, this invention adopts the following technical solutions: A linear sensor includes a ferromagnetic layer, an antiferromagnetic layer, and a heavy metal layer. The ferromagnetic layer is disposed on the upper surface of the antiferromagnetic layer, and the heavy metal layer is disposed on the upper surface of the ferromagnetic layer as a protective layer; when working, a Wheatstone bridge structure is adopted, and the direction in which the magnetization on each bridge arm is pinned forms an angle of 45° with its long axis direction, and the magnetoresistance response trends of the magnetoresistances on adjacent bridge arms to the same external magnetic field are opposite; a unidirectional current pulse with a set amplitude is passed through both ends of the magnetoresistance to induce exchange bias.

[0004] Further, the protective layer is heavy metal Ta or Pt, the middle ferromagnetic layer is NiFe or NiCo, and the bottom antiferromagnetic layer is FeMn, PtMn or IrMn.

[0005] Further, the thickness of the protective layer ranges from 3 nm to 10 nm, the thickness of the middle ferromagnetic layer ranges from 5 nm to 20 nm, and the thickness of the bottom antiferromagnetic layer ranges from 4 nm to 20 nm.

[0006] Further, the Wheatstone bridge structure includes a Wheatstone structure with a four-electrode layout and a Wheatstone structure with a five-electrode layout.

[0007] Further, in the Wheatstone structure with a four - electrode layout, by designing the wire layout to adjust the current flow direction of the magnetoresistance on the bridge arm, a regulation current pulse is input only once on the electrodes at the opposite ends, so as to complete the induced regulation of the exchange bias and the direction of the major axis on each bridge arm of the entire bridge at an angle of 45°.

[0008] Further, the Wheatstone structure with a five - electrode layout is to split one electrode of the four - electrode Wheatstone bridge into two separate electrodes, and a wire layout is introduced between the magnetoresistance units of the bridge arm.

[0009] A preparation method of a linear sensor includes the following steps: Select a substrate and perform pre - treatment on the substrate; Perform photolithography on the substrate, expose it with a mask of the functional layer, and obtain the designed pattern after development; Using thin - film deposition technologies such as magnetron sputtering, grow a ferromagnetic layer, an antiferromagnetic layer, and a heavy - metal layer in sequence, and ultrasonically clean the excess - covered thin film with an organic solvent, and finally obtain a functional layer with a pre - designed pattern. Use a second mask to cover with photoresist and develop to obtain the designed pattern of the electrode material; Using thin - film deposition technologies such as magnetron sputtering, grow the electrodes required for the sensor, and ultrasonically clean the excess electrode thin film with an organic solvent to obtain an AMR sensor.

[0010] Further, the pre - treatment of the substrate: Using a silicon wafer, Kapton or glass as the substrate, clean the substrate successively with acetone, alcohol, and deionized water, perform ultrasonic cleaning treatment respectively, and dry it with pure nitrogen.

[0011] Further, after obtaining the AMR sensor, use a current pulse to induce the regulation of the magnetization direction of the magnetoresistance on the bridge arm of the sensor.

[0012] Further, the typical order of magnitude of the regulated current density of the sensor is between 1×10 10 A m -2 and 1×10 12 A m -2 .

[0013] Compared with the prior art, the present invention has the following technical effects: The present invention aims to provide a simple operation method for inducing exchange bias. Without the aid of an external magnetic field or a magnet, by utilizing the spin - Hall effect and Rashba effect of heavy metals and multi - layer films, combined with the assistance of current Joule heat, as well as the anisotropy and exchange - bias effect of strip - shaped magnetoresistance, only a unidirectional current pulse (abbreviation: current pulse) is used to achieve the pinning of the exchange bias at an angle of 45° with the major axis.

[0014] Through the selection of materials for each layer of the sensor film, the design of the growth sequence, and the design of the backend Wheatstone bridge, the present invention realizes the regulation of the sensor with a Wheatstone full-bridge structure in a non-magnetic environment with only one current pulse. This greatly improves the growth efficiency of the magnetic sensor, reduces the regulation steps, and simplifies the sensor regulation program. Description of the Drawings

[0015] Figure 1 Sensor multi-layer film structure and spin current direction.

[0016] Figure 2 Induction of exchange bias of the exchange-biased magnetoresistive strip by pulsed current.

[0017] Figure 3 Response of the hard axis and easy axis of the magnetoresistive strip to the magnetic field in the initial state.

[0018] Figure 4 Schematic diagram of the Rxy(PHE) test for the single magnetoresistive regulation effect.

[0019] Figure 5 Response of Rxy to the magnetic field after the current SOT effect regulation.

[0020] Figure 6 Four-electrode Wheatstone bridge structure layout.

[0021] Figure 7 Five-electrode Wheatstone bridge structure layout. Detailed Embodiment

[0022] The present invention is further described below with reference to the accompanying drawings: When current flows through heavy metals such as Pt, Ta, and W, due to the spin Hall effect and Rashba effect, spin currents will be formed inside the heavy metals or at the two-dimensional interface, separating into two spin-polarized currents with opposite spin directions and flowing directions. The generated spin-orbit torque (SOT) can induce the formation of exchange bias or regulate its direction. The magnetization direction of the multi-layer film can be induced by using the spin current generated by the multi-layer film itself.

[0023] The multi-layer film structure used in this example is a silicon wafer, antiferromagnetic IrMn, ferromagnetic NiFe, and heavy metal Ta. As Figure 1 shown, when current flows through the multi-layer film, the spin and flowing direction generated by the multi-layer film. Combining the characteristic that the spin Hall angles of IrMn and Ta are opposite, by designing the multi-layer film structure, the spin currents generated by the two are concentrated at the interface where the exchange bias (EB) is generated, realizing the full utilization of the spin current of the multi-layer film and improving the efficiency of current regulation of EB.

[0024] The experiment adopted the technology of magnetron sputtering coating to achieve the growth of high-quality multilayer films.

[0025] Example 1: A linear sensor, the sensor multilayer film of which is characterized by a structure of a working layer and a heavy metal layer. In order to obtain a more stable and linear output, the working circuit is usually designed as a Wheatstone bridge structure.

[0026] The direction in which the magnetization on each bridge arm is pinned forms a 45° angle with its long axis direction, and the magnetoresistance response trends of the magnetoresistances on adjacent bridge arms to the same external magnetic field are opposite, that is, when the magnetoresistance of a certain bridge arm increases with the increase of the magnetic field in a certain direction, the magnetoresistances of the adjacent two bridge arms decrease with the increase of this magnetic field.

[0027] The main structure of the magnetoresistive strip for operation is composed of structures such as antiferromagnet, ferromagnet, and heavy metal protective layer. The upper protective layer is a heavy metal such as Ta and Pt, and the thickness ranges from 3 nm to 10 nm. The intermediate ferromagnetic material is a magnetoresistive material with strong anisotropy such as NiFe and NiCo, and the thickness ranges from 5 nm to 20 nm. The bottom antiferromagnetic material is FeMn, IrMn, etc., and the thickness ranges from 4 nm to 20 nm.

[0028] Example 2: A preparation method of a linear sensor, the specific steps are as follows: Step 1: Clean the silicon wafer successively with acetone, alcohol, and deionized water, perform ultrasonic cleaning treatment for 5 minutes respectively, and dry it with pure nitrogen.

[0029] Step 2: Adopt the lift-off technology. First, perform photolithography on the silicon substrate, spin-coat a layer of photoresist on the substrate. First, spin-coat at 600 revolutions per minute for 20 seconds to evenly spin the photoresist and fully cover the substrate; then spin-coat at 4000 revolutions per minute for 40 seconds to thin the photoresist to an appropriate thickness by using the centrifugal force of high-speed rotation.

[0030] Step 3: Put the coated silicon substrate into an oven and dry it at 115 °C for 20 minutes to cure the photoresist.

[0031] Step 4: Use the designed functional layer mask plate to perform an exposure treatment on the substrate for 6.5 seconds in an ultraviolet light environment. After development, the designed functional layer pattern can be obtained.

[0032] Step 5: Utilize thin film deposition technologies such as magnetron to successively grow the required AMR sensor films. And use organic solvents such as acetone and alcohol to perform ultrasonic cleaning on the excess films, and finally obtain a functional layer with the same pattern as the mask plate.

[0033] Step 6: Adopt the lift-off technology, use the second mask plate, repeat the above steps 1 to 4, and after development, obtain the designed electrode pattern.

[0034] Step 7: Use thin film deposition techniques such as magnetron sputtering to grow the electrodes required for the sensor, and ultrasonically clean the excess electrode film with organic solvents such as acetone. Thus, the growth of the sensor is completed.

[0035] Test the magnetic field response of the magnetoresistance of the hard axis (H) and the easy axis (E) of the obtained magnetoresistance stripes. The test results are as Figure 2 shown. It can be seen that under the initial conditions, there is no exchange bias in both the hard axis and the easy axis of the magnetoresistance stripe.

[0036] Utilize the spin Hall effect, Rashba effect, etc. of Ta and IrMn in the multilayer film. When a current flows through the magnetoresistance stripe from left to right, as Figure 3 shown, the generated spin-orbit torque (SOT) will induce an exchange bias to form on the left side of the current flow direction of the magnetoresistance.

[0037] To facilitate the test of whether the exchange bias has been regulated to a direction at a 45° angle with the magnetoresistance stripe, we design the magnetoresistance stripe as a "cross" structure, as Figure 4 shown. Test the change of Rxy by applying a varying magnetic field at different angles, that is, the change of the planar Hall (PHE) resistance value. The current flow direction and the positive and negative poles of the nanovoltmeter during the test are as Figure 4 shown. We mainly focus on the 45° and 135° angle directions. For example, when the exchange bias rotates to the 45° angle, by applying a varying magnetic field along the 45° direction, the curve of Rxy will be biased relative to H = 0 Oe, and when applying a magnetic field along the 135° angle direction, the Rxy curve is symmetric about H = 0 Oe; based on this, it can be judged whether the exchange bias has been successfully regulated to the 45° angle or the 135° angle direction.

[0038] When a regulation current pulse of 105 mA is applied along the Figure 4 Iin to Iout direction, respectively test the magnetic field response of Rxy at 45° and 135°. The results are as Figure 5 shown. It can be seen that the Rxy response curve with a magnetic field applied along the 45° direction has a bias of about 300 Oe relative to H = 0 Oe, and the Rxy response curve with a magnetic field applied along the 135° direction is symmetric about H = 0 Oe. It can be determined that the current pulse with an amplitude of 105 mA has successfully induced a 45° angle exchange bias of the exchange-bias-free magnetoresistance.

[0039] Since the range of thin film materials, thicknesses, and geometric dimensions that the invention can adopt is relatively wide, for the convenience of reference and characterization, the regulated current density is selected as the parameter to characterize the regulation conditions. In this example, the regulated current density is 2×10 11 A / m -2 .

[0040] To obtain an AMR sensor with good linearity and sensitivity, we designed the magnetoresistance as a push-pull Wheatstone bridge structure. To adapt to different sensor production conditions, we designed two circuit structures for selection.

[0041] First, the designed four-electrode Wheatstone bridge is as Figure 6 shown. According to the multilayer film structure designed in this instance experiment, when looking in from the current input port, the exchange bias will form exchange bias pinning on the left side in the direction of the regulated pulse current. Therefore, as Figure 6 shown, a regulated current pulse is passed from electrode D to electrode B, while electrodes A and C are kept open. Through the shunting of the regulated current by the resistors on the upper and lower parallel bridge arms, by designing the layout of the conductors in the bridge arm, the flow direction of the regulated current through the magnetoresistance unit is controlled, and one regulated current pulse can achieve the formation of exchange bias pinning on all four bridge arms. The obtained exchange bias direction is as Figure 6 shown. When testing the magnetic field along the horizontal or vertical direction, the response changes of the magnetoresistance of adjacent bridge arms on the Wheatstone bridge to the magnetic field are opposite (i.e., when the resistance of one side increases, the resistance of the other side decreases). This bridge structure requires a regulated current pulse with a high amplitude and a high-quality thin film, that is, the resistance values of the four bridge arms cannot vary too much, which is convenient for the equal division of the regulated current to control the regulated current density. However, this circuit scheme can greatly simplify the procedure of welding electrodes after the traditional five-electrode regulation.

[0042] Another design of the five-electrode Wheatstone bridge is as Figure 7 shown. By designing the wire layout in the bridge arm, the flow direction of the magnetoresistance regulated current in the horizontal (vertical) direction is adjusted to the same direction, which is convenient for achieving the regulation of the exchange bias direction of the magnetoresistance on all bridge arms with only one current pulse. After passing a regulated current with a certain amplitude from electrode B to electrode C direction, the formation of 45° exchange bias pinning as Figure 7 shown can be achieved. This circuit design has a higher fault tolerance for the magnetoresistance and reduces the requirement for the amplitude of the regulated current pulse.

[0043] It should be noted that the above examples are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Any modifications or equivalent replacements made to the specific implementation manners of the present invention without departing from the spirit and scope of the present invention shall be covered within the protection scope of the claims of the present invention.

Claims

1. A linear sensor, characterized in that: It includes a ferromagnetic layer, an antiferromagnetic layer and a heavy metal layer, wherein the ferromagnetic layer is arranged on the upper surface of the antiferromagnetic layer, and the heavy metal layer is arranged on the upper surface of the ferromagnetic layer as a protective layer; a Wheatstone bridge structure is adopted during operation, the magnetization on each bridge arm is pinned in a direction that is 45° to the direction of its long axis, and the magnetoresistance response trends of the magnetoresistances on adjacent bridge arms to the same external magnetic field are opposite; a unidirectional current pulse of a set amplitude is passed through both ends of the magnetoresistance to complete the induction of exchange bias.

2. A linear sensor according to claim 1, characterized in that: The protective layer is heavy metal Ta or Pt, the middle ferromagnetic layer is NiFe or NiCo, and the bottom antiferromagnetic layer is FeMn, PtMn or IrMn.

3. A linear sensor according to claim 1, characterized in that: The thickness of the protective layer ranges from 3 nm to 10 nm, the thickness of the middle ferromagnetic layer ranges from 5 nm to 20 nm, and the thickness of the bottom antiferromagnetic layer ranges from 4 nm to 20 nm.

4. A linear sensor according to claim 1, characterized in that: The Wheatstone bridge structure includes a four-electrode Wheatstone structure and a five-electrode Wheatstone structure.

5. A linear sensor according to claim 4, characterized in that: The Wheatstone structure with four electrodes adjusts the current flow direction of the magnetic resistance on the bridge arm by designing the wire layout. Only one control current pulse is input to the electrodes at the opposite ends to complete the induced control of the exchange bias on each bridge arm of the entire bridge at an angle of 45° to the long axis.

6. A linear sensor according to claim 4, characterized in that: The five-electrode Wheatstone structure is obtained by splitting one electrode of the four-electrode Wheatstone bridge into two separate electrodes by introducing a wire layout between the magnetoresistive units of the bridge arms.

7. A method for preparing a linear sensor, characterized in that: A linear sensor according to any one of claims 1 to 6, comprising the following steps: Selecting a substrate and pre-treating the substrate; The substrate is subjected to photolithography, exposed using a mask of the functional layer, and the designed pattern is obtained after development; Using thin film deposition techniques such as magnetron, the ferromagnetic layer, antiferromagnetic layer and heavy metal layer are grown in sequence, and the excess film is ultrasonically cleaned using an organic solvent to finally obtain a functional layer with a pre-designed pattern; Using a second mask, performing photoresist covering and developing to obtain a designed electrode material pattern; The electrodes required for the sensor are grown using thin film deposition techniques such as magnetron, and the excess electrode film is ultrasonically cleaned with an organic solvent to obtain an AMR sensor.

8. The method for preparing a linear sensor according to claim 7, characterized in that: Pretreatment of substrate: Use silicon wafer, Kapton or glass as substrate, clean the substrate with acetone, alcohol and deionized water in sequence, perform ultrasonic cleaning respectively, and blow dry with pure nitrogen.

9. The method for preparing a linear sensor according to claim 7, characterized in that: After obtaining the AMR sensor, the magnetoresistance on the bridge arm of the sensor is induced and regulated by using a current pulse.

10. The method for preparing a linear sensor according to claim 7, characterized in that: The current density of the sensor is typically on the order of 1×10 10 A m -2 Up to 1×10 12 A m -2 between.

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